NCERT Solutions for Class 11 Biology Digestion and Absorption [Old Book]

NCERT
SOLUTIONS
CLASS - 11th

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Class : 11th
Subject : Biology
Chapter : 16
Chapter Name : Digestion And Absorption

Q1 Choose the correct answer among the following:
(a) Gastric juice contains
(i) pepsin, lipase and rennin
(ii) trypsin lipase and rennin
(iii) trypsin, pepsin and lipase
(iv) trypsin, pepsin and renin

(b) Succus entericus is the name given to
(i) a junction between ileum and large intestine
(ii) intestinal juice
(iii) swelling in the gut
(iv) appendix

Answer. (a):(i) Pepsin, lipase, and rennin
Gastric juice contains pepsin, lipase, and rennin. Pepsin is secreted in an inactive form as
pepsinogen, which is activated by HCI. Pepsin digests proteins into peptones. Lipase breaks down
fats into fatty acids. Rennin is a photolytic enzyme present in the gastric juice. It helps in the
coagulation of milk.

(b):(ii) Intestinal juice Succus entericus is another name for intestinal juice. It is secreted by the
intestinal gland. Intestinal juice contains a variety of enzymes such as maltase, lipases,
nucleosidases, dipeptidases, etc.

Q2 Match column I with column II

Answer.

Q3 Answer brie y:
(a) Why are villi present in the intestine and not in the stomach?
(b) How does pepsinogen change into its active form?
(c) What are the basic layers of the wall of alimentary canal?
(d) How does bile help in the digestion of fats?

Answer.

(a) The mucosal wall of the small intestine forms millions of tiny nger-like projections known as
villi. These villi increase the surface area for more ef cient food absorption. Within these villi,
there are numerous blood vessels that absorb the digested products of proteins and carbohydrates,
carrying them to the bloodstream. The villi also contain lymph vessels for absorbing the products
of fat-digestion. From the bloodstream, the absorbed food is nally delivered to each and every
cell of the body. The mucosal walls of the stomach form irregular folds known as rugae. These help
increase the surface area to volume ratio of the expanding stomach.
(b) Pepsinogen is a precursor of pepsin stored in the stomach walls. It is converted into pepsin by
hydrochloric acid. Pepsin is the activated in the form of pepsinogen
(c) The walls of the alimentary canal are made up of four layers. These are as follows:

(i) Serosa is the outermost layer of the human alimentary canal. It is made up of a thin layer of
secretory epithelial cells, with some connective tissues underneath.
(ii) Muscularis is a thin layer of smooth muscles arranged into an outer longitudinal layer and an
inner circular layer.
(iii) Sub-mucosa is a layer of loose connective tissues, containing nerves, blood, and lymph
vessels. It supports the mucosa.
(iv) Mucosa is the innermost lining of the lumen of the alimentary canal. It is mainly involved in
absorption and secretion.
(d) Bile is a digestive juice secreted by the liver and stored in the gallbladder. Bile juice has bile
salts such as bilirubin and biliverdin. These break down large fat globules into smaller globules so
that the pancreatic enzymes can easily act on them. This process is known as emulsi cation of
fats. Bile juice also makes the medium alkaline and activates lipase.

Q4 State the role of pancreatic juice in digestion of proteins.

Answer. Pancreatic juice contains a variety of inactive enzymes such as trypsinogen,
chymotrypsinogen, and carboxypeptidases. These enzymes play an important role in the digestion
of proteins.
Physiology of protein-digestion The enzyme enterokinase is secreted by the intestinal mucosa. It
activates trypsinogen into trypsin.
Enterokinase
T rypsinogen T rypsin + I nactivepeptide
⟶

Trypsin then activates the other enzymes of pancreatic juice such as chymotrypsinogen and
carboxypeptidase. Chymotrypsinogen is a milk-coagulating enzyme that converts proteins into
peptides.
Trypsin
Chymotrypsinogen(I nactive) Chymotrypsin(Active)
⟶
Chymotrypin
P roteins P eptides
⟶

Carboxypeptidase acts on the carboxyl end of the peptide chain and helps release the last amino
acids. Hence, it helps in the digestion of proteins.

Carboxypeptidase
P eptides Smallerpeptidechain + Aminoacids
⟶

Thus, in short, we can say that the partially-hydrolysed proteins present in the chyme are acted
upon by various proteolytic enzymes of the pancreatic juice for their complete digestion.

Q5 Describe the process of digestion of protein in stomach.

Answer. The digestion of proteins begins in the stomach and is completed in the small intestine.
The digestive juice secreted in the gastric glands present on the stomach walls is called gastric
juice. The food that enters the stomach becomes acidic on mixing with this gastric juice. The main
components of gastric juice are hydrochloric acid, pepsinogen, mucus, and rennin. Hydrochloric
acid dissolves the bits of food and creates an acidic medium so that pepsinogen is converted into
pepsin. Pepsin is a protein- digesting enzyme. It is secreted in its inactive form called pepsinogen,
which then gets activated by hydrochloric acid. The activated pepsin then converts proteins into
proteases and peptides.
Pepsin
P roteins P roteases + P eptides
⟶

Rennin is a proteolytic enzyme, released in an inactive form called prorennin. Rennin plays an
important role in the coagulation of milk.

Q6 Given the dental formula of human beings

Answer. The dental formula expresses the arrangement of teeth in each half of the upper jaw and
the lower jaw. The entire formula is multiplied by two to express the total number of teeth. The
dental formula for milk teeth in humans is:
2102
× 2 = 20
2102

Each half of the upper jaw and the lower jaw has 2 incisors, 1 canine, and 2 molars. Premolars are
absent in milk teeth.
The dental formula for permanent teeth in humans is:
2123
× 2 = 32
2123

Each half of the upper jaw and the lower jaw has 2 incisors, 1 canine, 2 premolars, and 3 molars.
An adult human has 32 permanent teeth.

Q7 Bile juice contains no digestive enzymes, yet it is important for digestion. Why?

Answer. Bile is a digestive juice secreted by the liver. Although it does not contain any digestive
enzymes, it plays an important role in the digestion of fats. Bile juice has bile salts such as
bilirubin and biliverdin. These break down large fat globules into smaller globules so that the
pancreatic enzymes can easily act on them. This process is known as emulsi cation of fats. Bile
juice also makes the medium alkaline and activates lipase.

Q8 Describe the digestive role of chymotrypsin. What two other digestive enzymes of the same
category are secreted by its source gland?

Answer. The enzyme trypsin (present in the pancreatic juice) activates the inactive enzyme
chymotrypsinogen and chymotrypsin.
Trpsin
Chymotrypsinogen Chymotrypsin
⟶

(Inactive)

The activated chymotrypsin plays an important role in the further breakdown of the partially-
hydrolysed proteins.
Protien Peptides
Chymotrypin

⟶

The other digestive enzymes of the same category are trypsinogen and carboxypeptidase. These
are secreted by the same source-gland, pancreas. Trypsinogen is present in an inactive form in the
pancreatic juice. The enzyme enterokinase — secreted by the intestinal mucosa — activates
trypsinogen into trypsin.
Trypsinogen Trypsin +Inactive peptide.
Enterokinase

⟶

Q9 How are polysaccharides and disaccharides digested?

Answer. The digestion of carbohydrates takes place in the mouth and the small intestine region of
the alimentary canal. The enzymes that act on carbohydrates are collectively known as
carbohydrases.
Digestion in the mouth: As food enters the mouth, it gets mixed with saliva. Saliva — secreted by
the salivary glands — contains a digestive enzyme called salivary amylase. This enzyme breaks
down starch into sugar at pH 6.8.
Starch Maltose + Isomaltose +Limit dextrins Salivary amylase continues to act in
Salivary amylase

pH6.8

the oesophagus, but its action stops in the stomach as the contents become acidic. Hence,
carbohydrate-digestion stops in the stomach.
Digestion in the small intestine:
Carbohydrate-digestion is resumed in the small intestine. Here, the food gets mixed with the
pancreatic juice and the intestinal juice. Pancreatic juice contains the pancreatic amylase that
hydrolyses the polysaccharides into disaccharides.
Similarly, the intestinal juice contains a variety of enzymes (disaccharidases such as
maltase, lactase, sucrase, etc.). These disaccharidases help in the digestion of
disaccharides. The digestion of carbohydrates is completed in the small intestine.
Mallose
Maltose 2 Glucose
⟶

tactasc
Lactose Glucose + Galactose
⟶

Sucrose Sucrase

⟶
Glucose + Fructose

Q10 What would happen if HCL were not secreted in the stomach?

Answer. Hydrochloric acid is secreted by the glands present on the stomach walls. It dissolves bits
of food and creates an acidic medium. The acidic medium allows pepsinogen to be converted into
pepsin. Pepsin plays an important role in the digestion of proteins. Therefore, if HCL were not
secreted in the stomach, then pepsin would not be activated. This would affect protein digestion.
A pH of about 1.8 is necessary for proteins to be digested. This pH is achieved by HCL.

Q11 How does butter in your food gets digested and absorbed in the body?

Answer. Digestion of fats: Butter is a fat product and gets digested in the small intestine. The bile
juice secreted by the liver contains bile salts that break down large fat globules into smaller
globules, so as to increase their surface area for the action of lipase. This process is referred to as
emulsi cation of fats. After this, the pancreatic lipase present in the pancreatic juice and the
intestinal lipase present in the intestinal juice hydrolyse the fat molecules into triglycerides,
diglycerides, monoglycerides, and ultimately into glycerol.
Fats Pancreatic

lipuse
Triglycerides + Diglycerides

Diglycerides and monoglycerides Glycerol
Lipases
Fatty acids +
→

Absorption of fats : Fat absorption is an active process. During fat digestion, fats are hydrolysed
into fatty acids and glycerol. However, since these are water insoluble, they cannot be directly
absorbed by the blood. Hence, they are rst incorporated into small droplets called micelles and
then transported into the villi of the intestinal mucosa.
They are then reformed into small microscopic particles called chylomicrons, which are small,
protein-coated fat globules. These chylomicrons are transported to the lymph vessels in the villi.
From the lymph vessels, the absorbed food is nally released into the bloodstream and from the
bloodstream, to each and every cell of the body.

Q12 Discuss the main steps in the digestion of proteins as the food passes through different parts
of the alimentary canal.

Answer. The digestion of proteins begins in the stomach and is completed in the small intestine.
The enzymes that act on proteins are known as proteases.
Digestion in the stomach: The digestive juice secreted in the gastric glands present on the
stomach walls is called gastric juice. The main components of gastric juice are HCI, pepsinogen,
and rennin. The food that enters the stomach becomes acidic on mixing with this gastric juice. The
acidic medium converts inactive pepsinogen into active pepsin. The active pepsin then converts
proteins into proteases and peptides. The enzyme rennin plays an important role in the
coagulation of milk.

Digestion in the small intestine: The food from the stomach is acted upon by three enzymes
present in the small intestine — pancreatic juice, intestinal juice (known as succus entericus), and
bile juice. Action of pancreatic juice Pancreatic juice contains a variety of inactive enzymes such as
trypsinogen, chymotrypsinogen, and carboxypeptidases. The enzymes are present in an
inactivated state. The enzyme enterokinase secreted by the intestinal mucosa activates
trypsinogen into trypsin.
Trypsinogen Enterokinase

⟶
Trypsin +Inactive peptide

The activated trypsin then activates the other enzymes of pancreatic juice. Chymotrypsinogen is a
proteolytic enzyme that breaks down proteins into peptides.
Tnnsin
Chymotrypsinogen Chymotrypsin
⟶

(Inactive)

Protien Peptides
Chymotrypsin

⟶

Carboxypeptidases act on the carboxyl end of the peptide chain and help in releasing the last
amino acids.

Action of bile juice : Bile juice has bile salts such as bilirubin and biliverdin which break down
large, fat globules into smaller globules so that pancreatic enzymes can easily act on them. This
process is known as emulsi cation of fats. Bile juice also makes the medium alkaline and activates
lipase. Lipase then breaks down fats into diglycerides and monoglycerides. Action of intestinal
juice Intestinal juice contains a variety of enzymes. Pancreatic amylase digests polysaccharides
into disaccharides. Disaccharidases such as maltase, lactase, sucrase, etc., further digest the
disaccharides. The proteases hydrolyse peptides into dipeptides and nally into amino
acids.Pancreatic lipase breaks down fats into diglycerides and monoglycerides. The nucleases
break down nucleic acids into nucleotides and nucleosides.

Q13 Explain the term thecodont and diphyodont.

Answer. Thecodont is a type of dentition in which the teeth are embedded in the deep sockets of
the jaw bone. Ankylosis is absent and the roots are cylindrical.

Examples include living crocodilians and mammals. Diphyodont is a type of dentition in which
two successive sets of teeth are developed during the lifetime of the organism. The rst set of
teeth is deciduous and the other set is permanent. The deciduous set of teeth is replaced by the
permanent adult teeth. This type of dentition can be seen in humans.

Q14 Name different types of teeth and their number in an adult human.

Answer. There are four different types of teeth in an adult human. They are as follows:
(i) Incisors : The eight teeth in the front are incisors. There are four incisors each in the upper jaw
and the lower jaw. They are meant for cutting.
(ii) Canines : The pointy teeth on either side of the incisors are canines. They are four in number,
two each placed in the upper jaw and the lower jaw. They are meant for tearing.
(iii) Premolars : They are present next to the canines. They are eight in number, four each placed
in the upper jaw and the lower jaw. They are meant for grinding.
(iv) Molars : They are present at the end of the jaw, next to the premolars. There are twelve
molars, six each placed in the upper jaw and the lower jaw.
Hence, the dental formula in humans is
2123
× 2 = 32
2123

This means each half of the upper jaw and the lower jaw has 2 incisors, 1 canine, 2 premolars, and
3 molars. Hence, an adult human has 32 permanent teeth.

Q15 What are the functions of liver?

Answer. Liver is the largest and heaviest internal organ of the body. It is not directly involved in
digestion, but secretes digestive juices. It secretes bile which plays a major role in the
emulsi cation of fats.